(a) Undercolor removal for image quality--It is well known that three subtractive color primary inks, if printed in superposition, should in purest principle combine to form black. (The most common subtractive primary colors are cyan, magenta and yellow--constituting the "CMY" color space or system.) It is almost equally well known that real subtractive color primaries when superposed produce an off-black that to some observers is sometimes muddy-looking and otherwise undesirable.
Many workers' reports, including the Dillinger patent mentioned above, have pointed out that this effect, sometimes perceived as undesirable, can be avoided by substituting a true black ink for the common fraction of three subtractive primaries--that fraction sometimes being known as "undercolor". Such substitution has been implemented in various ways in numerous products.
(b) Undercolor removal for liquid control--To achieve vivid colors in inkjet printing with aqueous inks, and to substantially fill the white space between addressable pixel locations, ample quantities of ink must be deposited. Doing so, however, requires subsequent removal of the water base--by evaporation (and, for some printing media, absorption)--and this drying step can be unduly time consuming.
In addition, if a large amount of ink is put down all at substantially the same time, within each section of an image, related adverse bulk-colorant effects arise: so-called "bleed" of one color into another (particularly noticeable at color boundaries that should be sharp), "blocking" or offset of colorant in one printed image onto the back of an adjacent sheet with consequent sticking of the two sheets together (or of one sheet to pieces of the apparatus or to slipcovers used to protect the imaged sheet), and "cockle" or puckering of the printing medium. Various techniques are known for use together to moderate these adverse drying-time effects and bulk- or gross-colorant effects.
One known way to reduce the amount of liquid deposited on a page, in the course of printing an image, is to substitute black ink for a common fraction of three superposed subtractive primaries. This strategy is particularly useful and important in one special case in which liquid deposition is particularly heavy--namely, when deep shadow areas of an image occupy large areas of a printed sheet, calling for relatively very large amounts of colorant.
This substitution strategy is actually the same substitution mentioned in subsection (a) above, in regard to an arguable improvement of color appearance. Hence the substitution of one black inkdrop for three chromatic subtractive primaries provides two quite separate, recognized benefits simultaneously.
(c) Undercolor balance for richer shadows--To some observers, however, a composite black is sometimes richer and deeper than a black produced by printing with black ink. At any rate it is particularly useful when used as an add-on to black ink, since black ink alone produces a visual effect that is not opaque enough--or in graphics jargon not "snappy" enough--for a critical eye. The color inks are available anyway in a color printer, and can be used to augment the opacity of black ink.
Thus it is known to retain some undercolor black in addition to solid black ink--giving a total amount of inking that is, for instance, up to three hundred or even four hundred percent of "full inking". In other words, at each pixel of an image region that is so treated such a known system may deposit an average of three or four inkdrops.
To deposit "an average of three" drops, it is possible to print one drop of black and for example on the average two-thirds of a drop of each of three subtractive chromatic primaries. This can be accomplished, for instance, by printing one drop of all three of those chromatic inks in two out of every three pixels throughout the region.
In inkjet printing, such inking produces a desirably dense black effect but does suffer from the drawback of depositing a large amount of liquid on the page. One existing product which operates in this way is discussed in subsection (e) below.
(d) Undercolor balance for dithered image quality--In the above-mentioned Perumal and Dillinger patent, moreover, it is also shown that under some circumstances substitution of black ink for superposed color inks can amount to too much of a good thing. In particular when black inkdrops are distributed sparsely in a region of a printed image, they produce a distinctly grainy, harsh appearance which is often--depending on the subject matter of the illustration--undesirable.
This grainy appearance also may give the misimpression that the printing device which formed the image on a printing medium is not capable of fine resolution. For these various reasons Perumal and Dillinger teach a technique for replacing black inkdrops with matched quantities of three subtractive color inks--in other words, precisely the opposite of the black substitution discussed above.
In their patent, the point of departure is assumed to be an input data set expressed in terms of the hue-plus-gray color space taught in the Dillinger patent. In that way of representing colors, at the outset it is a given that as much undercolor black as possible is expressed strictly in terms of black inkdrops.
The teaching of Perumal and Dillinger moderates this approach, providing a softer, smoother image texture in relatively lightly printed areas, because it replaces each widely-scattered dead-black inkdrop with three less-widely-scattered color inkdrops. Because the drops are still somewhat widely scattered, a viewer tends not to notice the chromatic character of these drops.
Perumal and Dillinger go still further, also replacing secondary inkdrops with individual primary drops--thereby yet further distributing the inking in terms of area. In their process the black component of the color vector is replaced first (leaving secondary components for replacement only if additional replacement is feasible) because, as their patent puts it, the black dots are "darker and therefore more offensive with respect to attempting to represent continuous tone in the lighter shades."
In the Perumal patent, however, the techniques disclosed have the effect of making the black-to-color (and secondary-to-primary) substitution only in areas where the darker inkdrops would otherwise be scattered somewhat sparsely, which is to say in highlight areas of an illustration. Deep shadow areas are left rendered by black (and secondary) ink, according to the teachings of Perumal et al., who teach a novel method which they call "dithering on a color vector".
Dithering is a rendition technique often regarded as best adapted to images that have relatively large or simple image-element structures, or indeed to business graphics and like illustrations characterized by solid blocks or fields of color. Thus the Perumal method of establishing a consistent undercolor strategy for both shadow and highlight regions is most suited for illustrations that have extensive solid-color fields or at least lack important fine detail.
Moreover, the Perumal method of dithering on a color vector is useful mainly, or entirely, in systems that are designed according to the hue-plus-gray scheme and that therefore do have color vectors--with pure-black components--on which to dither. Working in a nonCMY color space, Perumal applies a replacement of gray and secondary colors that is linear; this latter constraint means that some significant black replacement by undercolor occurs even in somewhat dark shadow regions, whereas a significant amount of black ink is allowed to remain even in highlights.
(e) Undercolor balance with dither, in products--In one established product of the Hewlett Packard Company, undercolor removal 152 (FIG. 12, upper left view) is provided only when the undercolor fraction k, plotted along the abscissa, is about sixty percent; and the removal fraction 152 increases at first very slowly for another ten percent or so. Undercolor removal exceeds twenty percent only after the undercolor fraction passes about eighty or eighty-five percent, so that it is in fact confined strongly to the deepest gray shades--and for such regions the amount of undercolor removal and corresponding black generation increases very rapidly with increasing undercolor fraction.
The fraction of undercolor removed 152 may be compared with the total amount 151 present, and also with remainder 154-156 (lower left view). The latter curve represents what is left after the removed fraction 152 is subtracted from the total 151, and as can be seen it has three distinct features: a long segment 154 ascending at nearly the rate of the initial total undercolor 151, followed by a peak 155, and finally a more abruptly descending tail 156.
Thus the remainder 154-156 does not always increase or decrease as the undercolor fraction k increases--such behavior would be called monotonic; rather it sometimes increases and sometimes decreases. This characteristic is due to the interplay between the linearly rising total undercolor 151 and the removed fraction 152--which starts much later but then increases rapidly.
Black generation 153, shown in the upper right view in comparison with the initially present total undercolor 151, is derived separately. This amount 153 of black to be added appears also in comparison with the remaining undercolor 154-156.
Since the remaining undercolor 154-156 and the newly generated black 153 are both to be printed, their sum 157-159 (lower right view) is of particular interest. It represents the total amount of ink that will go down on the printing medium.
What is noteworthy here is, first, that the total inking 157-159, too, fails to be monotonic. In fact, due to the interplay between the peak of the remainder curve 154-156 and the black curve 153 sharply rising in the same region--skewing the sum to the right--the total inking 157-159 has a peak 158 which is even sharper than that in the remainder curve 154-156.
Also of note is the vertical scale in the lower right view, ranging not from zero to unity as in the other views but rather from zero to three. The total initial undercolor 151 in this graph accordingly appears much shallower, since the combined inking approaches three hundred percent of normal (I.e., single-dot inking)--before falling back off nearly to two hundred percent.
In physical terms these graphed data thus represent a printing system that routinely uses almost three inkdrops per pixel, but not in the darkest shadows--rather in a region of lesser shadow intensity around seventy-five to ninety percent initial undercolor. Such a system, though capable of quite excellent performance, is susceptible in particularly dark regions to overinking problems of the sort discussed earlier.
Equally or more serious is its possible susceptibility to nonlinear perceptual effects, particularly if ink-and-printing-medium interactions do not proceed exactly as contemplated in design. In such cases the perceived darkness or blackness of what is printed on the medium may actually be greater at the seventy-five-to-ninety-percent undercolor level than at one hundred percent, or indeed at any level above ninety percent.
In other words, what should be midshadow tones may appear darker than what should be deepest-shadow tones. Perceptible lightening may occur where regions of an image should appear darkest, so that such regions instead appear lighter than the immediately adjacent areas that are--by the logic or the illustration--less deeply shaded.
This susceptibility to reentrant or nonmonotonic tonal scaling arises partly from the nonmonotonic total inking 157-159. It arises more particularly from the fact that the peak 158 of the total-inking function is sharply contoured, and is narrowly and awkwardly sandwiched between two confining features: (1) a rather high threshold for initiation of undercolor removal--at about sixty percent, as previously noted for the upper left view in FIG. 14--and (2) the rightward-skewing black-generation function 153, which rises most rapidly starting at about seventy-five percent.
Once again, the difficulty discussed here is only a susceptibility. In practical operating environments, however, it can take on the appearance of a seemingly temperamental response to what should be minor perturbations such as ink or paper batches, atmospherics etc.
(f) Undercolor balance with error diffusion--Other artisans have disclosed an approach which serves when more photograph-like images are involved--I.e., when the input image is a continuous-tone illustration. In such a case the rendition technique preferred for maintaining resolution, and for avoiding the distinct patterning that often accompanies dither techniques, is known as "error diffusion".
The above-mentioned patent of Motta et al. teaches an approach to undercolor treatment which is integrated into a following rendition stage that employs error diffusion. Accordingly their method is more suited to continuous-tone images--particularly such images with details that might be blurred or eradicated by a dither technique.
In a preferred implementation of the Motta technique, no undercolor removal occurs--and the amount 253' of black ink K.sub.out used remains at zero--unless and until the undercolor fraction or in Motta's phrase "gray component" reaches a point GC.sub.1 (FIG. 13) said to be 105/255 of full black, or about forty-one percent. Starting at that gray-component (undercolor) value the amount 253 of black ink K.sub.out used to represent gray increases (from zero) quite abruptly.
It thereafter continues rather steeply to a point at which it joins and follows the forty-five-degree identity line 251 (corresponding to the total initial undercolor 151 in FIG. 12), at about ninety percent undercolor and above. Beyond that point all black is represented 253" by black ink K.sub.out.
In general the Motta approach appears to work very well and to provide a major stride forward in the art of inkjet printing, and accordingly it is not intended to criticize the worthwhile innovations of Motta et al. Still it may be mentioned that their system would seem possibly susceptible to an overly abrupt onset of the undercolor replacement in negotiating the sharp corner in the response curve at GC.sub.1 --and this in a midrange of grays where any abrupt change in coloration characteristics may be particularly noticeable to a critical eye. Moreover, that teaching is limited to the error-diffusion environment.
In addition, if the Motta data are analyzed in the same manner as the earlier-product data of FIG. 12 discussed in subsection (e) above, it may be seen that the Motta system too may be strongly nonmonotonic in total inking, and with an even more abrupt response anomaly near the top of the darkness scale. More specifically, Motta's total-inking response would appear to rise linearly to about fifty percent of normal inking--at about forty percent of initial undercolor--and then to traverse a tall hump in the response, to about one hundred seventy percent of normal ink at roughly sixty to eighty percent of initial undercolor.
The inking response would next fall to about one hundred five percent of normal ink at ninety-two percent initial undercolor--but then rise linearly to about one hundred ten percent of total ink at one hundred percent of initial undercolor. The total-ink curve thus has not one but two reversals, the second being quite sharp.
Accordingly, while the system when properly tuned by inks, papers etc. should perform excellently, and in particular should be very well behaved as to tonal response, it may be susceptible to seemingly erratic behavior in perhaps extraordinary circumstances not all parameters are strictly in accordance with design specifications. Under such conditions, as explained in subsection (d) above, a nonmonotonic total-inking function, and particularly a relatively narrow or sharp one, may yield nonmonotonic perceptual response.
(g) Conclusion--As can now be seen, the art of inkjet printing has heretofore not attained an ideal balance between reliable enjoyment of the benefits of undercolor replacement with black, in deep-shadow regions, and the drawbacks of such replacement in highlight regions. Such an ideal balance has been particularly elusive for operating modes that do not employ error diffusion, or systems that do not adhere to the hue-plus-gray paradigm and therefore are not amenable to dithering on a color vector. Thus important aspects of the technology used in the field of the invention remain amenable to useful refinement.